Directional fluctuating asymmetry in certain morphological characters as a pollution indicator: Tigris catfish (Silurus triostegus) collected from the Euphrates, Tigris, and Shatt al-Arab Rivers in Iraq

The relationship between pollution levels in river sediment and fluctuating asymmetry of resident silurid fish species, Silurus triostegus Heckel, was assessed. Eight bilateral body proportions were analyzed in 1,500 fish from eight river locations. Sediment pollution was measured based on the analysis of heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) and organochlorine pesticides (DDT, DDD, DDE, chlordane, dieldrin, and lindane). The mean quotient approach (mERMq) was used to characterize sediment toxicity, which ranged from low to moderate levels for heavy metals and from low to severe for organochlorides. Variation was noted in the measurements of fish body morphometrics among the sampling locations, which suggested responses to local environments. Levels of asymmetry were positively correlated with both organochlorine pesticides and heavy metals across locations. These results suggest that fish asymmetry variations could be useful for estimating stress caused by organic toxicity based on the mERMq approach.